A common method of earth formation logging is resistivity logging in which an electric current is injected or induced in the earth formation and the resulting voltage is determined to provide a resistivity log, which is a measure of the resistivity of the formation as a function of depth. However, a resistivity log seldom reads the true formation resistivities, which are the resistivities of the undisturbed formation in the so-called virgin zone away from the borehole. The resistivity log is influenced by disturbing effects such as the presence of the borehole, invasion of wellbore fluid into the formation (mud-filtrate invasion) and the presence of adjacent earth layers (the so-called shoulder beds). In case one of these effects dominates over the other, so-called correction charts may be used to correct the log for the dominating effect. However, in most cases the disturbing effects are simultaneously present and interweaved in such a way that adding of the individual corrections does not lead to the true formation resistivity.
U.S. Pat. No. 5,446,654 discloses a method of recovering a resistivity profile of an earth formation from a resistivity log by inversion processing via iterated forward modelling. In this method a modelled resistivity profile is initialized and subsequently rectangularized to simulate the different layers of the earth formation. The rectangularized modelled resistivity profile is input to a logging tool simulator to provide a modelled resistivity log. The rectangularized modelled resistivity profile is then corrected (if necessary) in dependence on a discrepancy between the modelled resistivity log and the actual resistivity log.
A drawback of the known method is that no invasion of wellbore fluid into the formation is taken into account and that therefore the obtained resistivity profile does not accurately represent the formation resistivity.
It is an object of the present invention to provide an improved method of determining electric resistivity of an earth formation, whereby account is taken of invasion of wellbore fluid into the formation surrounding the wellbore.